1. Field of the Invention
In injectors for injecting fuel into the combustion chambers of internal combustion engines, very high fuel pressures occur. On the one hand, the injectors for injecting the fuel are embodied with the necessary fatigue strength; on the other hand, however, the attempt should be made wherever possible to relieve the components of such an injector, for example the nozzle needles, from the high pressures that occur in order to reduce the mechanical strain.
2. Description of the Prior Art
DE 198 35 494 A1 relates to a pump/nozzle unit for feeding fuel into a combustion chamber of directly injected internal combustion engines. A pump unit is provided for building up an injection nozzle. Furthermore, a control unit with a control valve is provided, which is embodied as an A-valve opening outward, as well as a valve actuation unit for controlling the build-up of pressure in the pump unit. The valve actuation unit is embodied as a piezoelectric actuator for the purpose of achieving short response times of the pump/nozzle unit.
DE 37 28 817 C2 relates to a fuel injection pump for an internal combustion engine.
The control valve member is comprised of a valve shaft, which forms a guide sleeve and slides in a channel, and a valve head, which is connected thereto, is oriented toward the actuation device, and whose sealing surface cooperates with the surface of the control bore forming the valve seat. On its circumference, the valve shaft has a recess whose axial span extends from the mouth of the fuel supply line to the beginning of the sealing surface on the valve head that cooperates with the valve seat. In the recess, a surface is formed which is subjected to the pressure of the fuel supply line and is the same size as a surface of the valve head that is subjected to the pressure of the fuel supply line when the control valve is closed. In this manner, the valve is pressure balanced when it is closed. Furthermore, the guide sleeve contains a spring which loads the control valve in the direction of its open position.
In the currently used injector designs for high-pressure accumulation chamber applications (common rail), the injection nozzle is relieved of pressure for the closing process. A 3/2 seat-slide valve is used for this purpose. During the closing phase of the valve, the fact that the high-pressure inlet is still open can coincide with the fact that the leakage oil outlet has already opened. By virtue of a delayed pressure relief on the nozzle needle, the pressure does not decrease rapidly enough and the closing phase is delayed. The short circuit between the common rail inlet and the leakage oil outlet occurring during the closing phase decreases the efficiency of the injector considerably.
With the injector for injecting fuel into the combustion chambers of directly injected internal combustion engines, which is proposed according to the invention, the closing phase of the injector control part, which can move in the injector housing, can be used to increase the pressure against the nozzle needle so as to achieve a pressure build-up there which encourages the closing movement. For this purpose, a compression spring is admitted in a hollow chamber provided below the leakage oil slide valve and a piston element is mounted in a movable manner. During the closing phase, the overflowing fuel from the high-pressure accumulation chamber can act on an end face of the piston element provided below the leakage oil slide valve. As a result, this piston element acts on a compression spring element, which rests against a plate surface of the nozzle needle.
The pressure build-up in the piston element only occurs when, through actuation of the control part, its closing edge on the high-pressure side is inserted into a seat face in the housing of the injector and closes the inlet, which branches off from the valve chamber and leads to the nozzle chamber.
The connection of the valve chamber, into which the common rail inlet empties, to the outlet-side leakage oil chamber, which occurs for a brief time during the closing phase, can be enlisted in order to use the high pressure, which prevails for a brief time by way of the high-pressure inlet, to produce a closing motion of the nozzle needle in order to reduce its closing time. In this manner, the volume of the overflowing fuel caused by short circuit that briefly occurs can be kept within limits, which permits improvement of the overall efficiency of the injector proposed according to the invention. Moreover, the fuel injection quantity to be measured can be metered with considerably greater accuracy because closing times as well as opening pressures can now be determined with significantly greater precision.
In the reverse case of the opening of the nozzle needle, by providing the piston element, which is associated with the nozzle needle, an opening of the nozzle needle can only be performed in the case of a higher opening pressure. Only after an opening pressure has been achieved in the nozzle chamber, which exceeds the forces acting on the nozzle needle by way of the piston element and/or the sealing spring, does an opening of the injection nozzle occur. Thus, the injection process that can be achieved using the embodiment proposed according to the invention is substantially more precise because the build-up of injection pressure can initially wait until the injection of the injection quantity into the combustion chamber of the internal combustion engine occurs.